Hybrid J-T cooler has been the most commonly used 4K cryocooler in space detectors. Although resistance of the J-T valve deeply affects the performance of the J-T cooler, few researches have been down especially on the J-T orifice. Because the J-T process is quite complicate, the dimensionless resistance coefficient is defined and deduced using dimensional analysis method to evaluate the pressure drop of the J-T orifice. Then, experimental research is carried out and a series of J-T orifices are experimentally tested. Then, the experimental correlation equations of the resistance coefficient at 4.2K are achieved based on the results of dimensional analysis and experiments.